Microwave attenuator switch



Jan. 23, 1962 P. J. sFERRAzzA 3,018,454

MICROWAVE ATTENUATOR SWITCH Filed May 16, 1955 nil l N VE TO R Pf TER d.JFf/QRA ZZA United States Patent() 3,018,454 MICROWAVE ATTENUATOR SWITCHPeter J. Sferrazza, Wantagh, N.Y., assignor to Sperry Rand Corporation,a corporation of Delaware Filed May 16, 1955, Ser. No. 508,711 3 Claims.(Cl. S33- 98) This invention relates generally to microwave transmissionapparatus, and more particularly, is concerned with apparatus forattenuating, modulating, or switching microwave energy.

1t is well known that certain commercially available substances known asferrites, when utilized in the propagation of microwaves, exhibit thephenomenon known as Faraday effect. Thus, when a piece of ferrite isplaced in a wave guide propagating microwaves, and the ferrite issubjected to a strong magnetic field in the direction of propagation ofthe microwaves, the rotation of the plane of polarization of theincident wave results. The amount of rotation is linearly related to thelength of the ferrite and the strength of the magnetic field. Thisphenomenon exhibited by the ferrites and other ferromagnetic materialshas been utilized to provide microwave switches, modulators, orattenuators by utilizing means in a circular wave guide which isselectively responsive to energy propagated in only one polarizationplane. an attenuator card extending across a circularwave guideattenuates energy propagated in a mode in which the electric field isparallel to the attenuator card, while energy having its electric vectorperpendicular to the attenuator card is substantially unaffected. Thus,by varying the plane of polarization of the incident energy by changingthe magnetic field applied to the ferrite, variable attenuation of theincident energy can be achieved. However, such known prior art devicesutilizing the Faraday effect of ferrite on microwave energy have beenconfined to the circular wave guide type of transmission line, becausethe symmetry of circular wave guide permits energy to be propagated inan infinite number of planes. Since most microwave transmission linesystems employ rectangular wave guides, such known devices require theuse of rectangular-to-circular transition elements at either end of theferrite device. Such transition devices add materiall1y to the length ofthe ferrite unit, its complexity and cost,

and adversely affect performance.

It is the general object of this invention to avoid the foregoinglimitations in and objections to the prior art practice by the provisionof a microwave attenuator switch utilizing ferrite rotation which ismore compact in design, simpler in construction, and less expensive tobuild.

Another object of this invention is the provision of apparatus that canbe utilized as an attenuator, a modulator, or a switch.

Another object of this invention is to provide apparatus using ferriterotation which can be used in a rectangular wave guide transmission linesystem and requires no rectangular-to-circular wave guide transitionsections.

These and other objects of the invention which will become apparent asthe description proceeds are achieved by apparatus comprising a sectionof rectangular wave guide in which is centrally mounted a ferritemember. A magnetizing coil surrounds the wave guide in the region of theferrite member for applying a magnetic field extending in a directionparallel to the longitudinal axis of the wave guide. By varying themagnetizing current through the coil, the degree of Faraday rotation ofthe incident energy in the wave guide can be controlled. Since therectangular wave guide propagates energy in the dominant or TEN mode inwhich the electric vector is perpendicula-r to the broad walls of thewave guide, and acts as a cut-off wave guide to any mode of propagationin which the electric vector extends parallel to the broad For example,

3,018,454 Patented Jan. 23, 1962 ice walls of the rectangular waveguide, the rotational effect of the ferrite reduces the amount of energypropagated down to wave guide in the dominant TEN, mode.

For a better understanding of the invention, reference should be had tothe accompanying drawing, wherein:

FIG. l is an elevational View in section of a structure incorporatingthe principles of the present invention;

FIG. 2 is an end view of the embodiment shown in FIG. l;

FIG. 3 is an elevational View in section of a modified version of thepresent invention; and

FIG. 4 is an end view of the embodiment shown in FIG. 3.

Referring to FIGS. l and 2, the numeral 10 indicates generally a sectionof rectangular wave guide which may be part of a rectangular wave guidetransmission line coupling a source of microwave energy to a suitableutilization device or load. Positioned within the section of therectangular wave guide is a rod 12 of ferrite, or other materialexhibiting the Faraday effect as to microwave energy. The ferritecompounds are well-known in the art and are commercially available. Theyare composed of a magnetic oxide of iron plus other metallic oxides. Therod 12 is positioned centrally within the waveguide and supported bysuitable dielectric support members, as indicated at 14 and 16.

An axially extending magnetic field is applied to the ferrite rod 12 bymeans of a wire coil 18 connected across an exciting current source 20.A rheostat 22 in series with the source 20 is provided for varying thestrength of the magnetizing current in the coil. The coil 18 is woundaround the outside of rectangular wave guide section 10.

In operation, energy propagates in the rectangular wave guide in the TEmmode in which the electric vector is normal to the broad walls. When theenergy encounters the ferrite to which an axially extending magneticfield has been applied, rotation of the electric vector is effected byyan amount `depending upon the strength of the magnetic field for agiven length of ferrite rod. Rotation results in a component of theincident energy which extends parallel to the broad walls of therectangular wave guide. However, the component of energy having theelectric vector extending parallel to the broad walls, corresponding tothe TEM mode, cannot propagate along the wave guide, -since the waveguide is below cut-off for this mode. Thus, by varying the strength ofthe magnetic field applied to the rotator, the division of energybetween the TEN, and the TEM mode can be varied. By rotation, thecomponent of energy having the electric vector normal to the broad wallsis reduced, with the result that the energy propagated down the waveguide is substantially attenuated. By increasing the magnetizing currentto a value where the incident energy is rotated through substantially noenergy is propagated beyond the ferrite. Thus, the deviceV of FIGS. land 2 can be utilized as a variable attenuator or a switch.

In the embodiment of the invention above described in connection withFIGS. l and 2, the component of energy having the electric vectorextending parallel lto the broad walls of the wave guide is subjected tocontinuous internal reflection and attenuation by the ferrite rod andthe walls of the wave guide. In the embodiment of FIGS. 3 and 4, anattenuator card, indicated generally at 24, is provided extendingbetween the narrow walls of the wave guide section 10. To permit roomfor the attenuator card, the ferrite rod is positioned adjacent to abroad wall of the wave guide 10 and secured thereto. By introducing theattenuator card as shown in the modification of FIGS. 3 and 4, thecomponent of energy extending parallel to the broad walls, as producedby the rotation introduced by the ferrite member 24, is substantiallyattenuated. This reduces reflections and improves the matching of thisdevice to the line.

Instead of a variable D.C. magnetizing current, as provided by thebattery 20 and rheostat 22, an A.C. signal may be applied to themagnetic coil 18 to modulate the microwave energy propagated along therectangular wave guide at the frequency of the A.C. signal applied tothe coil 18.

From the above description of the invention, it will be seen that thevarious objects have been achieved by a ferrite rotator employing arectangular wave guide. The property of the rectangular wave guide ofpropagating only the dominant mode in which the electric vector isnormal to the broad walls of the wave guide is utilized to achieveattenuation or modulation of the microwave energy propagated along theline. This structure obviates the need for rectangular-to-circulartransition sections heretofore required in known prior art ferriterotators. While the invention has been described as utilizing coils forapplying an axially extending magnetic field through the ferrite rod, itwill be understood that other suitable means, such as permanent magnets,may be utilized for establishing the magnetic field.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus comprising a section of rectangular wave guide forpropagating microwave energy in the dominant TEM) mode only, a ferritemember positioned within the wave guide, an attenuator card extendingparallel to the broad walls of the Wave guide, the card being positionedin the region of the ferrite member, a wire coil surrounding the waveguide in the region of the ferrite member, means connected to the wirecoil for passing a magnetizing current therethrough to establish amagnetic field directed axially Within said waveguide in the regionoccupied by said ferrite member, said magnetic field having a strengthto magnetize said ferrite member to a condition that causes rotation ofthe plane of p-olarization of microwave energy propagating through saidwaveguide, whereby at least a component of the microwave energy has itselectric vector rotated parallel' to a broad wall ofsaid waveguide andis reflected thereby, said attenuator card serving to attenuate saidretiected energy, and means for varying the current through the wirecoil to control the rotation of the microwave energy by the ferritemember.

2. Apparatus comprising a section of rectangular wave guide forpropagating microwave energy in the dominant TEM, mode only, a ferritemember positioned within the wave guide, attenuator means comprising abody of dissipative material within said wave guide for attenuatingenergy components having the electric vector extending parallel to thebroad walls of the wave guide, and means for producing a magnetic fieldwhich is directed along the longitudinal axis of the wave guide in theregion of the ferrite member, said magnetic field having a strength tomagnetize said ferrite member to a condition that causes rotation of theplane of polarization of microwave energy propagating through saidwaveguide.

3. Apparatus comprising a section of wave guide having unequalcross-sectional dimensions whereby the wave guide is above cut-off forenergy polarized in one plane and below cut-ofi for energy polarized inanother plane, a plane of polarization rotator disposed within said waveguide section for rotating the plane of polarization of the electricvector of the microwave energy propagating in theV wage guide, andattenuator means comprising a body of dissipative material within saidwave guide for atteinuating the resulting component of energy having theelectric vector parallel to the broad walls of the wave guide.

References'Cited in the file of this patent UNITED STATES PATENTS2,542,185 Fox Feb. 20, 1951 2,628,278 zaleski Feb. 10, 1953 2,650,350Heath Aug. 25, 1953 2,719,274 Luhrs sept. 27, 1955 2,729,794 Cohn Jan.3, 1956 2,745,069 Hewitt May 8, 1956 2,748,353 Hogan May 29, 19562,802,183 Read Aug. 6, 1957 2,820,200 Du Pre Jan. 14, 1958 OTHERREFERENCES Article, Behavior and Applications of Ferrites in theMicrowave Region, by Fox et al pub. in Bell System Technical Journal,vol. 34, No. 1, Jan. 1955, pages 5-103, page 74 relied on,

